Iron oxide pigments are used in many, various industrial applications that involve solvents and media spanning the whole spectrum of polarity, from highly polar to nonpolar. For instance, red iron oxide pigments are used in concrete, which uses a polar media, water. Iron oxide pigments are also used in solvent-based coatings that use low-polarity solvents, such as toluene. Iron oxides are inherently hydrophilic. Consequently, they disperse fairly well in polar media. However, when they are used in nonpblar systems, surfactants or dispersants must be added to aid in the wetting and dispersion process. Such surfactants may cause problems in the formulation. In some industrial applications the hydrophilic pigment must be dispersed in a nonpolar medium, without the use of surfactants, such as when yellow iron oxide is used in certain plastic formulations. In such an instance, the yellow iron oxide must be "dry-milled" with the polymer matrix. The use of surfactants to aid in dispersing the iron oxide is impractical in that application.
Although iron oxide is the most consumed inorganic colorant, most commonly used dispersants are tailored to render the iron oxide either more hydrophilic (using silicate, aluminum hydroxide, sodium phosphates, or a mixture of aminoalcohols or their salts) or hydrophobic (using fatty acid alkanolamide esters, polycarbonate/polyurethane polymer, alkali siliconate silylalkylphosphate, dodecylbenzenesulphonates, or metal salts). Very few attempts have been made to produce a universally dispersible iron oxide pigment that is capable of being dispersed equally well in both polar and nonpolar media.
Where attempts have been made to render iron oxide universally dispersible, it usually has been by the use of post-production processing. Such an approach usually involves a redispersion of the dried pigment or presscake in water, followed by the addition of a desired concentration of a surfactant or a combination of two or more surfactants (one surfactant being useful in polar media and the other in nonpolar media). U.S. Pat. No. 4,165,239 discloses a process for treating iron oxide with carboxylic acid esters of tertiary alkylolamines to improve the dispersibility in organic and aqueous media. Japanese published application JP-94/232177 describes a process of coating inorganic pigments with modified polysiloxane to enhance the dispersibility in different media, but does not teach the use of single modified polysiloxane as being sufficient to enhance the dispersibility in both organic and aqueous media.
To the extent that universal dispersibility for certain pigments has been achieved by the use of such redispersion methods, such methods have added tremendous labor cost to the production of such pigments. Moreover, the addition of complex, expensive surfactants or dispersants has further raised the cost to produce such pigments.
Thus, a need exists for a fast, simple and economical way to prepare a universally dispersible pigment so that pigment particles may be easily dispersed in both polar and nonpolar solvents.